The present invention relates to an image processing method and an image forming method for obtaining a conversion function for obtaining analytical density representing an amount of dye from spectral densities of a color image.
Heretofore, various methods have been developed for improving color reproducibility of a color image in reading a color image by an image reading apparatus through reflected light and making a print on an image forming apparatus from spectral density of a color image read by the image reading apparatus in the wavelength regions of the primary colors of R (red), G (green) and B (blue). Generally, a color image is formed by cyan (C), magenta (M) and yellow (Y) dyes under the subtractive color mixing principle. FIG. 16 shows the spectral absorption characteristics of each dye and the spectral absorption characteristics of a color image formed by mixing these dyes. As can be seen from the spectral absorption characteristics of each dye, for example, the yellow dye absorbs not only light in the blue wavelength region which corresponds to the complemental color wavelength region of the yellow dye, but also slightly absorbs the light in the red wavelength region and the green wavelength region. Accordingly, when spectral densities of the yellow dye are measured, the spectral densities of db(Y), dg(Y) and dr(Y) are obtained. Likewise, when spectral densities of the magenta dye are measured, the spectral densities of db(M), dg(M) and dr(M) are obtained, and when spectral densities of the cyan dye are measured, the spectral densities of db(C), dg(C) and dr(C) are obtained. There is the following relationship among these spectral densities.
Db=db(Y)+db(M)+db(C) PA1 Dg=dg(Y)+dg(M)+dg(C) PA1 Dr=dr(Y)+dr(M)+dr(C) PA1 W.sub.kl : Ratio of secondary absorption density (density of l primary color) to main density (density of k primary color) of subtractive dye (k.noteq.l=r,g,b) PA1 .alpha.ij: Matrix coefficient PA1 a magenta image employing only a magenta dye and a cyan image employing only a cyan dye by an image forming apparatus, PA1 a step to obtain spectral density in each of red, green and blue wavelength regions through reflected light for each of the aforementioned yellow image, magenta image and cyan image, PA1 a step to obtain, by the use of a densitometer, analytical density for each of the yellow image in the blue wavelength region, the magenta image in the green wavelength region and the cyan image in the red wavelength region, and PA1 a step to obtain, from the spectral density obtained by the image reading apparatus and from the analytical density obtained by the densitometer, the conversion function that is used for obtaining the analytical density from the spectral density. PA1 a step wherein a mixed-dye image in which at least two kinds of dyes out of yellow dyes, magenta dyes and cyan dyes are mixed is made by the image forming apparatus, and PA1 a step to obtain a spectral density signal for each of red, green and blue of the aforementioned mixed-dye image through reflected light by the use of the aforementioned image reading apparatus. PA1 a step wherein a color image is read by an image reading apparatus through reflected light and thereby photometry densities for red, green and blue are obtained, PA1 a step to obtain analytical density from spectral density based on the conversion function for obtaining analytical density from spectral density obtained by the image processing method described in one of the aforementioned methods 1 to 13, and PA1 a step to determine, based on the analytical density mentioned above, an amount of yellow dyes, an amount of magenta dyes and an amount of cyan dyes all in an image forming apparatus, and thereby to make a print. PA1 a step to store in a memory medium the conversion function for obtaining analytical density from spectral density obtained by the image processing method described in one of the aforementioned methods 1 to 13 and PA1 a step to obtain analytical density from the spectral density based on the conversion function stored in the memory medium mentioned above. PA1 (A) forming a yellow image with a given amount of yellow dye alone, a magenta image with a given amount of magenta dye alone and a cyan image with a given amount of cyan dye alone; PA1 (B) obtaining spectral densities of each of the yellow image, the magenta image and the cyan image in the red, green and blue wavelength regions; PA1 (C) obtaining an analytical density of an yellow component in the blue wavelength region from the yellow image, an analytical density of a magenta component in the green wavelength region from the magenta image, and an analytical density of a cyan component in the red wavelength region from the cyan image; PA1 (D) forming a mixed-dye image by using at least two of the given amount of yellow dye, the given amount of magenta dye, and the given amount of cyan dye so that the mixed-dye image comprises at least two of a yellow component, a magenta component and a cyan component; PA1 (E) obtaining spectral densities of the mixed-dye image in the red, green and blue wavelength regions; PA1 (F) determining an analytical density of the yellow component in the blue wavelength region, an analytical density of the magenta component in the green wavelength region and an anlytical density of the cyan component in the red wavelenght in the mixed-dye image on the basis of the analytical densities of the yellow, magenta and cyan components in their complementary color wavelength regions obtained from the yellow, magenta and cyan images; and PA1 (G) arranging, for each of the yellow, magenta and cyan images and the mixed-dye image, the spectral densities in the red, green and blue wavelength regions obtained in the steps (B) and (E) so as to correspond to the analytical densities of the yellow, magenta and cyan components in their complimentary color wavelength regions obtained in the step (C) and (F), thereby obtaining a conversion function to obtain analytical densities of yellow, magenta and cyan components in their complementary color wavelength regions in a color image from spectral densities of the color image in the red, green and blue wavelength regions.
Each spectral density from db(Y) to dr(C) in the right side of the above formulas is called an analytical density. In contrast, each spectral density of Db, Dg and Dr in the left side of the above formulas is called an integral density.
The spectral densities of a color image obtained by an image reading apparatus are integral densities. That is, in the integral density Db in the blue wavelength region, an analytical density db(M) of magenta dye in the blue wavelength region and an analytical density db(C) of cyan dye in the blue wavelength region are added with the analytical density db(Y) of yellow dye in the blue wavelength region, wherein the magenta and cyan dyes are not a blue wavelength absorbing dye respectively, and, on the other hand, the yellow dye is a blue wavelength absorbing dye. Accordingly, although the analytical density db(Y) of yellow dye in the blue wavelength region represents an amount of the yellow dye, the integral density Db in the blue wavelength region is not equal to the analytical density db(Y) of yellow dye in the blue wavelength region. That is, the integral density Db in the blue wavelength region is not equal to the amount of the yellow dye which is the blue wavelength absorbing dye. Consequently, if the printing is conducted by an image outputting apparatus on the basis of the integral densities, color contamination may take place, resulting in that it may be difficult to reproduce a high quality image. Incidentally, the analytical density db(Y) of yellow dye in the blue wavelength region is called the primary density in the blue wavelength region, on the other hand, the analytical density db(M) of magenta dye and the analytical density db(C) of cyan dye in the blue wavelength region are called the auxiliary absorption densities in the blue wavelength region.
When an original image is a transmission document, a conversion function represented by Expression [1] is used for obtaining the analytical density from the spectral densities which are integral densities. ##EQU1## I.sub.r, I.sub.g, I.sub.b : Spectral density (Integral density) A.sub.r, A.sub.g, A.sub.b : Analytical density
This method is effective for a transmission original image. However, when reading a color image through reflected light, it does not always agree with Expression [1] due to an influence of scattering or the like. When reading a color image through reflected light, it is known that correct analytical density can not be obtained by a conversion function of Expression [1].
It is therefore known that a conversion function represented by Expression [2] is used for obtaining analytical density which is more accurate. ##EQU2## I.sub.r, I.sub.g, I.sub.b : Spectral density (Integral density) A.sub.r, A.sub.g, A.sub.b : Analytical density
This method can provide analytical density more effectively than the method of Expression [1]. However, even this method can not offer the correct analytical density because it is difficult to prepare the aimed color chip accurately, though many color chips representing analytical density determined by an image forming apparatus and a matrix coefficient is obtained by the use of the color chips. Further, considerable length of processing time is required for obtaining analytical density, which is a disadvantage.
Further, there is a problem that analytical density can not be obtained correctly through Expressions [1] and [2] especially in a high density area and a low density area, because a ratio of secondary absorption density to primary density of dyes for subtractive color process varies depending on an amount of the primary density of the subtractive dyes.
In addition, Japanese Patent O.P.I. Publication No. 88344/1992 discloses a method wherein a color reflection original image is read by an image reading apparatus, analytical density is obtained by using a ratio of secondary absorption density to primary density of color materials for subtractive color process and a coefficient indicating an index from spectral density of each primary color read by the image reading apparatus, and a print is made by an image forming apparatus based on the obtained analytical density.
In the method mentioned above, a color transmission-type original image made of photographic emulsions of a color reflection original image is prepared, and a coefficient indicating an index is obtained from the relation between transmission density of the color transmission-type original image and reflection density of the color reflection original image. However, the ratio of secondary absorption density to primary density of dyes for subtractive color process varies depending on the primary density of the dyes for subtractive color process due to an influence of scattering on the reflection original image. Therefore, the analytical density obtained in a low density area or a high density area in particular does not agree with an actual analytical density and thereby color reproduction in high fidelity can not be carried out, which is a problem.
In conventional technologies employing many chromaticities including those disclosed in Japanese Patent O.P.I. Publication No. 117567/1988, arbitrary input signals are given to an image forming apparatus in advance, various prints are made, and thereby the relation between the input signals for the image forming apparatus and the chromaticities (for example, chromaticity of L*a*b* color specification system, chromaticity of L*u*v* color specification system, and chromaticity of XYZ color specification system, etc.) is obtained in advance, then a color reflection original image is read by an image reading apparatus, chromaticities are obtained from photometry values of each primary color of the color reflection original image read by the aforesaid image reading apparatus, and thereby optimum input signals for the image forming apparatus are selected based on the obtained chromaticity.
However, in these conventional technologies employing chromaticities (for example, chromaticity of L*a*b* color specification system, chromaticity of L*u*v* color specification system, and chromaticity of XYZ color specification system, etc.), it is required to prepare many accurate standard color chips with known chromaticities in advance for obtaining a function (three-dimensional LUT or three-dimensional matrix) for converting the photometry values obtained by the image reading apparatus into chromaticities, and it is further required to convert the photometry values obtained by the image reading apparatus into chromaticities by using the function mentioned above and also to convert the chromaticities to input signals for the image forming apparatus. For obtaining the function (three-dimensional LUT or three-dimensional matrix) for converting from the chromaticity to an input signal for the image forming apparatus, there are required complicated processes wherein various kinds of color chips by means of input signals established in advance are prepared, each color chip is subjected to photometry by means of an image reading apparatus, and obtained photometry values are converted to chromaticity values. Further, in order to obtain these two functions correctly, standard color chips are required to be selected appropriately and the kinds of color chips prepared by the image forming apparatus need to be selected appropriately. The reason for this is that when either of a standard color chip for obtaining a chromaticity correctly from an image reading apparatus and a color chip for obtaining accurately an output value for an image forming apparatus is not prepared appropriately, correct color reproducing capability can not be obtained. In particular, when an individual image reading apparatus and an individual image forming apparatus are used in combination, chromaticity aimed by one apparatus is easily deviated from that aimed by the other apparatus, causing color reproducibility to be poor.
In addition, there is a problem that the time for computing process is long because of the combination of three-dimensional or higher computing processes in two steps including that from a photometry value to a chromaticity and that from a chromaticity to an input signal for an image forming apparatus.